A multi-pronged strategy

Standard vocabularies for some tasks Some kind of standard foundation is typically necessary to establish communication (protocols, for instance), and other levels of communication may in some cases benefit from further standardization.
Custom vocabularies for other tasks A huge amount of XML application is local - between only a couple of systems, or among a handful of individuals or organizations. These smaller groupings may find XML far more useful when structured to meet their particular needs, not those of a larger group.
And tools to move between them At some point, custom and standard vocabularies often meet. There may be layers of databases between them, or they may meet directly. In either case, some kind of processing is necessary to establish communications across these need-based boundaries.